As far as I know, the topic you mentioned goes back to a specific theory, the theory of light scattering. According to the theory, it seems that the spherical shape of nanoparticles can increase the scattering (not reflecting) of the incident photon which results in a better opportunity to receive the energy of the photon. This is a crucial subject in photovoltaic to receive photon's energy.
An important question. I think it is not only related to the shape, it is also related certainly to its density moreover. I think furthermore that the optoelectronic quality of this type of material has a non-negligible role in what you are asking about...